1. Field of the Invention
The present invention relates to articles made of a highly crystalline olefin polymer which have improved clarity and impact strength. More particularly, the present invention relates to articles such as containers obtained by the biaxial orientation blow molding of said olefin polymer.
2. Description of the Prior Art
As plastic containers, those made by conventional blow molding techniques from raw materials such as high density polyethylene, low density polyethylene and polyvinyl chloride, have heretofore been prevalent. However, there has recently been extensive interest in obtaining containers made by so-called biaxial orientation blow molding and this technique has been used to some extent with polyvinylidene chloride, polyvinyl chloride, polystyrene, crystalline homopolymers and copolymers of olefins such as polypropylene, etc. Techniques that use polypropylene or copolymers containing a major proportion thereof as raw material have attracted wide interest.
Biaxial orientation blow molding is a forming method wherein orientation is built up in a hollow body by extension in biaxial directions, i.e. longitudinal and lateral directions, so as to improve physical properties. One such method comprises extending a tubular plastic body formed by extrusion molding or injection molding in its longitudinal direction at a temperature within the region of the orientation temperature, and then further extending also in the lateral direction in a metal mold by blowing thereinto a pressurized fluid to shape the body into a desired form and this is known as the successive orientation method. Another method comprises forming a preliminary product by preforming the tubular body at orientation temperature in a metal mold and further blowing a pressurized fluid thereinto in a second metal mold having the desired final configuration, or blowing a pressurized fluid into said second mold while imparting longitudinal extension mechanically to shape into the desired form, and this is known as the simultaneous orientation method. Many methods have been proposed by, for example, Japanese Pat. Nos. 521,711 and 662,944, Japanese Pat. publication No. 47-1117, U.S. Pat. Nos. 3,294,885 and 3,244,778, etc. some of which have been used in practical application with various thermoplastic resins as the raw material therefor.
In particular, containers formed by biaxial extension blow molding of a highly crystalline homopolymer or copolymer of olefin such as polypropylene may be capable of exhibiting highly useful properties for packaging containers due to the great improvements in physical properties such as transparency, rigidity, impact strength and gas impermeability etc., compared to such containers made by conventional blow molding. However, among these properties, transparency is inferior to that of polyvinyl chloride containers made by conventional blow molding and impact strength is inferior to that of high density polyethylene containers so that further improvement is required even in the biaxially oriented containers.